The strtotime() functions handles most input formats.
So if your input field on the form is fm_date then
$dateStr = $_GET["fm_date"];
$thedate = date("Y-m-d", strtotime($dateStr))
would give you a date formatted as mysql stores them.
This would convert "10/25/02" or " 25 Oct 2002" to 2002-10-25.
I don't know where you are located, but dates with xx/xx/xx format are assumed to be mm/dd/yy so us Brits have a little more processing to do to switch to dd/mm/yy first.
hth